1. Field of the Invention
The present invention relates to a method for forming a lens from a high molecular weight, viscosity-specific polycarbonate having improved mechanical properties.
2. Description of the Related Art
Lenses and other articles manufactured at optical quality, have exacting demands for mold replication, high optical transmission and impact resistance. In the 2002 Polycarbonates publication, Brunelle and Kailasam describe how polycarbonate is prepared by the transesterification of a bisphenol-A with a carbonate. They explain how lower molecular weights provide a good balance between desirable mechanical properties and processability of the resin.
U.S. Pat. No. 5,212,280 describes diphenols which are useful in the condensation method of producing polycarbonate in the presence of phosgene. WO 95/14742 describes a carbonate blend polymer containing a low molecular weight component to improve processability. U.S. Pat. No. 4,699,745 describes a stretching ratio just prior to the granulation process for improving tensile strength in the resulting product. JP 60-23421 describes controlling operation of the molding machine to reduce distortions in large and thin articles. U.S. Pat. No. 6,613,869 addresses the molding of large and thin articles by specifying one of two blow molding techniques to make water bottles.
As the above survey indicates, many attempts have been made to chemically engineer low molecular weight resins for improved performance in injection molding applications. Furthermore, the high molecular weight resins have been relegated to blow molding and extrusion type processes which are not suitable for manufacturing solid articles requiring exacting mold replication. In general, the chemistry of a resin, including polymer type, nature of branching agents and presence of additives, dictated the available product-forming techniques. Accordingly, it would be desirable to injection mold optical lenses from high molecular weight, viscosity-specific polycarbonate, to obtain better mechanical properties, previously associated with extruded or blow-molded products only.